Minnesota's third-largest great blue heron rookery, site of a remarkable comeback over the past decade, suffered a setback recently, possibly at the hands of the same culprits who threatened it before.

The island rookery on Peltier Lake in Lino Lakes had 253 active nests a year ago, but when a volunteer visited the site in January, he counted only 203.

Some of the decline might be due to natural causes, but officials also suspect a wily adversary: raccoons.

Peltier Lake herons thrived in the early 1990s, with more than 1,000 nests at their peak. In the early 2000s, the number dropped, falling to about 25 in 2005, a decline that long puzzled officials.

"There was some concern [that] boating traffic around the island was a disturbance," said Carrol Henderson, nongame wildlife supervisor for the state Department of Natural Resources. "Until Andy got the cameras we were totally baffled."

DAVID JOLES &#x2022; djoles@startribune.com

The great blue herons had begun arriving from their winter sanctuaries a few weeks ago, a few weeks later than usual because of the harsh winter.

Andy was Andy Von Duyke, then a University of Minnesota graduate student who in 2004 installed and monitored surveillance cameras in the nest trees, with support from DNR funds and federal grants. Video showed raccoons appearing night after night in heron nests for snacks, wiping out all the chicks.

After the herons flew south for the winter that year, volunteers helped city and Anoka County workers install 3-foot-wide metal bands around the tree trunks. A no-wake zone around the island was also enforced.

Four chicks survived the next year, followed by 30 in 2006 and on up to 70 in 2011, as best as counters could tell. Nest numbers also improved.

The flashing stopped most of the raccoons. Some raccoon trapping was also done.

But when Wayne LeBlanc, 65, an area resident who does the annual nest and chick counts, visited the island in January, he found the 50-nest drop from the prior year.

"Some of this could be natural fluctuation," said the DNR's Henderson. "Last year there was late snow that lasted into the spring that could have caused some abandonment. The numbers can go up or down 10 or 20 percent" without raising concern.

Lino Lakes environmental coordinator Marty Asleson is one of those pointing the finger at raccoons: He said the animals circumvent the metal flashing by climbing neighboring or dead trees that make contact with the nest trees.

Henderson said he was unaware of any cases elsewhere of raccoons climbing into nests to take chicks.

On Thursday, a crew of residents and park workers set out to renew the campaign against the predators by fixing up flashing on trees. But the outing was canceled, for a positive reason.

The great blue herons had begun arriving from their winter homes a few weeks ago, a few weeks later than usual because of the harsh winter, and there was concern that the workers might scare off those that were rebuilding nests, Asleson said.

Dozens of the yellow-billed birds were visible on and near Peltier Island on Thursday.

"It's a great story that they have come back," said LeBlanc, one of the workers.

He said he's not worried about colony failure as long as 100 or more nests remain on the remote island, which is marked with signs against trespassing and "no-wake-zone" buoys.